Printed circuit boards are used in a wide variety of electronic devices, including computers and communications equipment, among other applications. The printed circuit boards are assembled by soldering components to conductive contacts on the board. The soldering may be accomplished by a number of different techniques, but serves to attach and electrically connect the components and conductive contacts to the board. Prior to soldering, however, a flux is usually applied to the surfaces intended to be joined. The flux chemically prepares the surfaces to receive the solder by removing and preventing the formation of stannous and stannic oxides on the surfaces. This promotes wetting and continuity of the solder at the interface with the circuit, thereby improving the quality and integrity of the electrical and mechanical connections between the adjoining surfaces.
After soldering the fluxed surfaces, and the assembly is cooled, the solder hardens and residual flux polymerizes to form deposits on the exposed surfaces. If allowed to remain on the printed circuit board, the residual flux can cause circuit failure due to stress corrosion resulting from exposure to temperature and humidity. In extreme cases, the flux residue can cause joint fatigue/cracking as there exists a coefficient of thermal expansion (CTE) mismatch between the flux polymer and the metal of the solder joint. Accordingly, the residual flux must be cleaned from the board.
Fluxes useful in electronics applications include rosin fluxes and water soluble fluxes. While rosin fluxes have been more traditionally employed, water soluble fluxes have gained interest due to their lower volatility and compliance with environmental requirements that are becoming ever more stringent. However, water soluble fluxes are much more corrosive than rosin fluxes, making them a less desirable flux for electronics applications. In particular, although water soluble fluxes may be removed with water, if they are not properly cleaned, the residual flux will degrade the treated electronic device. Specifically, the residual flux is chemically active, hydroscopic in nature, and will cause corrosion and etching of the metals, including the very electronic components that it was employed to help solder. The residual flux from water soluble fluxes puts long-term hardware performance at risk, negatively affects the performance of diode junctions, and makes costly field failures likely. Accordingly, rosin fluxes continue to be widely used in electronics applications.
One traditional category of rosin fluxes is rosin mildly activated (RMA) fluxes, which are water resistant. Because RMA fluxes are water resistant, their removal requires the use of organic solvents such as freon, trichloroethane, trichloroethylene, toluene, and isopropyl alcohol. However, in light of environmental restrictions which are becoming more stringent, removing RMA fluxes has become a daunting task. While discovering low volatile organic compound (VOC) cleaning chemicals has proved difficult, one proposed method of cleaning such fluxes includes “bomb proof” closed-loop systems for containing the VOCs resulting from the use of organic solvents. Although these “bomb proof” systems enable compliance with environmental regulations, they are very costly, requiring substantial investments in facility upgrades and new equipment.
In addition, RMA fluxes are stored in an isopropyl alcohol carrier. Because the isopropyl alcohol carrier evaporates rapidly when exposed to air, the pot life of traditional RMA fluxes has historically been limited.